Method and apparatus for retrieving information from a 3d storage medium

ABSTRACT

A method and apparatus for retrieving information from a three dimensional storage medium uses a three dimensional storage medium having an active medium capable of exhibiting first and second states, a data unit being represented by the ratio between the concentration of the first and second states in a given volume portion of the medium and a data sequence is represented by a sequence of data units. The active medium is irradiated with light as to concentrate light flux through a volume portion of the storage medium so as to generate in the volume portion a detectable non-linear optical response characteristic of the concentration ratio, which is detected and used for tracking.

FIELD OF THE INVENTION

The invention relates to methods and apparatus for reading informationfrom a 3D optical storage medium.

BACKGROUND OF THE INVENTION

It has been suggested in the art, to store information in threedimensional optical storage apparatuses. One of the problems to besolved in such systems is how to read information from a particularpoint without letting the reading light beam being distracted by thestorage medium positioned between the reading light source and the saidparticular point.

WO 01/73779 co-owned by the owner of the present invention, suggestsreading the information by two-photon absorption. In this method, theinformation stored in a particular point is characterized by theabsorption coefficient in a certain frequency ν, and the reading iscarried out with two light beams having frequencies ν₁ and ν₂, so thatν₁+ν₂=v (ν₁−ν₂=v is also possible). Only when the two light beamsintersect, the light may be absorbed and reading takes place. In all thepoints where the two beams do not intersect, there is no light offrequency ν, and therefore no reading. The storage medium should betransparent to light having a frequency ν₁, and also to light having thefrequency ν₂.

Regarding the storage means, it is suggested in WO 01/73779 to use amatrix carrying stilbene derivatives, having one characteristicabsorption in a given frequency when in the cis isomer and another, whenin the trans.

In optical storage media such as optical disks in general and DVDs inparticular, data is stored along tracks formed in the bulk of theoptical disk and is read by focusing a laser beam produced bysemiconductor diodes on to the tracks, while spinning the disk on itsaxis. The tracks generally comprise spiral tracks on which data iswritten and from which the data is read.

Obviously in order to retrieve data correctly it is essential that thereading head can locate and follow a desired track. In practice thisleads to two different kinds of tracking problem: skipping from onetrack to another and faithfully following a single track. For thepurpose of the present discussion, it suffices to observe that these twodifferent tracking problems require different solutions and to theextent that the method of tracking is relevant to the present invention,the present invention is concerned only with the second of the twoproblems.

U.S. Pat. No. 5,592,462 (Beldock) issued Jan. 7, 1997 entitled“Three-dimensional optical data storage and retrieval” discloses a threedimensional optical data storage and retrieval system having a threedimensional optical data storage medium and an apparatus for providingaccess to data stored on the medium. In accordance with one aspect, thedata storage medium includes a number of concentric shells each of whichhas a curvilinear data storage surface for storing data in a number ofsubstantially parallel data tracks. According to another aspect, thedata storage medium includes a number of data storage surfaces, whichare rotatable about a corm non axis, each data storage surface forstoring data in a number of substantially circular data tracks andhaving an optically transparent window, which transects each of the datatracks. In use, each shell or data storage surface is rotated about acommon axis and tracking is achieved by directing the reading beamthrough the optically transparent windows on to a data track ofinterest. Thus, this reference is not applicable to retrieving data froma solid optical storage medium wherein the data is stored in multiplelayers.

The manner in which CD and DVD reading head track a destination track isbased on focusing the reading spot on to the track and measuring theintensity of a reflected spot by position sensitive detectors. Thisallows calculation of the position of the reading spot and subsequentadjustment of the reading head's location based on the measured error.

U.S. 20010040844 published Nov. 15, 2001 (Sato et al.) entitled“Tracking servo apparatus of optical information recording andreproducing apparatus” discloses a tracking servo apparatus using thistechnique. Thus, reflection light obtained when a laser beam isirradiated onto a recording surface of an optical disc isphotoelectrically converted, thereby obtaining a photoelectricconversion signal. A tracking error signal showing an amount ofdeviation of an irradiating position of the laser beam for a track in adisc radial direction on the recording surface is generated by thephotoelectric conversion signal. A spherical aberration occurring in anoptical system is detected, a level of the tracking error signal iscorrected on the basis of the detection result, and the irradiatingposition of the laser beam is moved in the disc radial direction inaccordance with the level-corrected tracking error signal.

Likewise, U.S. Pat. No. 6,233,210 published May 15, 2001 (Schell; DavidL.) entitled “Optical drive error tracking method and apparatus”discloses a method and apparatus for obtaining a tracking error signalfor an optical disk player which is general across the various dataformats found in CD audio disks and DVDs. A photodetector having atleast four active areas is used to sense the reflected laser beam. Adifferential amplitude tracking error signal is generated by comparingthe signal strength in the different active areas.

These references are typical of known solutions for maintaining theread/write head in communication with a desired track using aphotodetector having multiple sections that serves as aposition-sensitive detector for detecting a component of the read/writelaser beam reflected from the surface of the optical disk.

For both CDs and DVDs, axial compensation translates to a focusingadjustment of the read/write beam.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method and system forretrieving information from a three dimensional storage medium.

It is a further object of the invention to correct tracking errors insuch a method and system where the read spot may drift in twoessentially orthogonal directions.

In accordance with a broad aspect of the invention, there is provided amethod for retrieving information from a three dimensional storagemedium, the method comprising:

-   -   using a three dimensional storage medium comprising an active        medium capable of being in two states, wherein a data unit is        represented by the ratio between the concentration of the first        and second of said two states in a given volume portion of said        medium and a data sequence is represented by a sequence of such        data units;    -   irradiating said active medium with light as to concentrate        light flux through a volume portion of said storage medium so as        to generate in said volume portion a detectable non-linear        optical response characteristic of said concentration ratio;    -   detecting said non-linear optical response to retrieve        information stored in said volume portion; and    -   tracking a data sequence for retrieving said data sequence in a        reproducible manner.

Within the context of the description and appended claims, the term“data unit” refers to a bit or symbol of a finite alphabet. Preferably,the data sequence is tracked via a tracking feedback signal forpositioning the light at a predetermined volume portion of the storagemedium.

According to a further aspect of the invention there is provided amethod for correcting tracking errors in an optical storage mediumhaving multiple tracks arranged in different layers of the opticalstorage medium, the tracking comprising:

-   -   (a) directing a reading spot that is nominally focused on to a        track in the optical storage medium,    -   (b) continually moving the reading spot in axial and radial        directions,    -   (c) receiving a signal having an amplitude which varies        according to respective offsets from the track in radial and        axial directions,    -   (d) using the received signal to determine a direction of a        respective offset from the track in radial and axial directions,        and    -   (e) adjusting a location of the reading spot accordingly.

Any active medium known in the art is suitable for use according to thepresent invention. Some non-limiting examples to active media are thosedescribed in WO 01/73779 and in U.S. Pat. No. 5,268,862, both of whichare incorporated herein by reference, stillbene derivatives, andazobenzene derivatives.

The active medium is preferably embedded in a supporting matrix, forinstance, as a dopant or, when the supporting matrix is a polymer, as amonomer co-polymerized with the supporting matrix. The supportive matrixshould be transparent for the light irradiated on it by the method ofthe invention and to the light generated by the non-linear opticalprocess. Non-limiting examples of supportive matrices suitable for useaccording to the present invention are polyethylene, polypropylene,polycarbonate, and polymethylmetacrilate (PMMA).

Typically, the data is stored in a binary mode, so that theconcentration ratio representing one digit is 1:0 and the concentrationratio representing the other digit is 0:1. Here, 1 and 0 are notabsolute values but rather should be interpreted as the highest andlowest concentrations that may be achieved during the writing process,which is not discussed herein.

As an alternative to storing the data in binary mode, other schemes maybe devised where the there are more than two states of the media (e.g.completely in isomerics states A or B, in the thermal equilibrium orclose to it, in one of a multitude of photo stationary states and more).The size of the alphabet used in the encoding-decoding process dependson the encoding-decoding method used and on the signal separation andthe signal resolution (signal to noise ratio) of the system. Manyencoding-decoding methods are known in the art, DC free and run lengthlimited encoding-decoding are non-limiting examples of such families ofcodes.

The size of the volume portion from which a data unit is retrievedaccording to the invention is the size of the light spot wherein theflux is large enough to generate a detectable non-linear reaction.Generally, smaller spot sizes may compensate for weaker lightintensities. Therefore, working with spots having a radius of less than30 μm is advisable, and spots having a radius equal to or smaller thanthe wavelength of the irradiated light is preferable.

Small spots allow the use of cheaper light sources. However, it may bebeneficial to work at high intensity of a given light source, even if anon-linear response is detected at lower intensities. This is so becauseabove some intensity, due to saturation effects, the response is nolonger sensitive to the intensity, and the reading is also not sensitivethereto. This way noise may be reduced from the measurement.

Detection of the signal requires its separation from other light signalsthat may exist in the environment. Such separation may be achieved byany method known for this purpose in the art, such as directing thenon-linearly generated signal into a direction where it is the onlysource for light of its frequency by satisfying phase matchingconditions; filtering the light through a filter, prism, grating,polarizers, etc.; using phase sensitive detection, lock-in amplifier, abox-cars, and/or gated averaging method. All the available methods maybe applied whether the beams irradiating the active medium are collinearor not.

Non-linear optical processes are very sensitive to the flux of light. Asthe flux varies as a function of volume element when a beam of light isfocused through matter, a dramatic increment in the effective efficiencyoccurs when approaching the focal point. The effective result of this isthat with the appropriate photon flux, the process occurs only at thelocus of maximal power, that is the focal point. When using a singlelight source at a single wavelength, this focal point isstraightforwardly defined as the location where the beam waist isminimal.

When using more than one wavelength the most efficient way to achievethe same effect of high localization is by overlapping the focal pointof all photon sources involved in the relevant process. When differentfoci are not exactly overlapping, the same process occurs, albeit lessefficiently, in the volume generated by the crossing of the effectivevolume elements generated by the foci of all light sources. To gainadditional effective efficiency in a one-wavelength process higherfluxes can be achieved through the use of more than one light sourceusing the same methods as for the multi-wavelength case.

The superposition principle allows one to consider a single beam as aplurality of beams whose respective foci are trivially located in thesame location. Therefore within the context of the present invention andthe appended claims where reference is made to two or more intersectinglight beams, it is to be understood that a single light beam may giverise to a non-linear process analogous to the overlapping of two or moremonochromatic beams, and this is encompassed by reference in the claimsto the intersection of two or more light beams.

One family of non-linear optical responses suitable for use according tothe present invention is a multi-photon fluorescence, such as, but notlimited to, two-photon fluorescence.

Non-limiting examples of non-linear optical responses related to a Xprocess, are four wave mixing processes such as Stimulated RamanScattering, Coherent Anti-Stokes Raman Scattering (CARS), Raman inducedKerr effect, and degenerate four-wave mixing. Similar X⁽⁵⁾ processes andhigher are also known in the art and may be used according to thepresent invention.

According to another aspect of the present invention there is providedan apparatus for retrieving information from a three dimensional storagemedium by generating a non-linear optical response of said storagemedium, detecting said non-linear optical response and analyzing andprocessing it. Such an apparatus includes, in order to generate anddetect a non-linear optical response, at least one light source, whichin some cases (such as CARS) must be coherent; a detector for detectinglight, which is different in at least one characteristic from the lightprovided by said light sources. In this context, examples of lightcharacteristics are the light wavelength, polarization, and propagationdirection. And means for tracking, i.e. process the signals receivedfrom the medium to get a tracking feedback signal and correct thelocation of the read spot accordingly. The apparatus may also includemeans, known per se in the art for analyzing and processing detectedsignals and retrieving information therefrom. These may comprise meansfor digitizing the detected signal, such as an A/D converter, and analgorithmic error detection means, such as error detector code runningon a computer or on an electronic chip.

A light source according to the present invention may be an active lightsource like a laser, or a passive light source like a mirror. A beamsplitter, for example, may be considered as two (passive) light sources.

According to one embodiment of the invention the data sequences arearranged as layers within the medium, each layer consisting of a spiraltrack of the respective data sequences, where the medium is shaped as adisk, and rotated around its axis by the apparatus. The purpose of theinvention is to track the spiral track corresponding to a required datasequence in r and z coordinates when the disk rotates. It is assumedthat the track suffers limited amount of run-out both in r (radialrun-out) and z (axial ran-out) coordinates. Such distortions can occurin the event that the axis of rotation is slightly off the disk centerand slightly non-parallel to the disk plane normal, such that the dataspiral moves relative to the reading spot while the disk rotates. Theinvention enables tracking the data spirals by calculating a trackingerror signal that is used as feedback for the servo-mechanisms thatcontrol the r and z position.

The basic tracking principle is to perpetually move (modulate) thereading spot in a periodical path around its nominal current position(traveling the r-z plane by two orthogonal functions of time). Thismodulation causes a modulation in amplitude and phase of the read signalthat depends on the position of the reading spot relative to the dataspiral. This dependence is used to determine the tracking error.

As a simplified example of the way the tracking algorithm calculates anerror signal, consider a 2-D case. Assume the z coordinate is fixed suchthat the lasers spot is focused at the proper height and there is noaxial run-out. As the reading spot propagates along the track, thespot's radial position is modulated in the radial direction so that thespot is half the time in (towards the center of the disk) and half thetime out relative to the track (i.e. r<r₀ half of the time and r>r₀ halfof the time). It should be noted that the offset relative to trackcenter has to be small to ensure that signal is still detected with asignal to noise ratio that is high enough for other functions such assymbol detection or synchronization to be accomplished. If the signalhas a fixed average and the tracking is perfect, than the average of the‘in’ signal (signal detected when spot is ‘in’ relative to the track) isequal to the ‘out’ signal. If the spot's position begins to diverge fromthe track's position e.g. because of eccentricity of the disk, theexpansion of the spiral or some other reason, then the differencebetween the ‘in’ and ‘out’ parts of the modulated signal, out-in, isnegative if there were a small ‘run-out’ or positive if there was‘run-in’.

Two main factors determine the frequency of the modulation. It should behigh enough to be able to respond to fast changes of the relativelocation of the track and the spot, but low enough to average enoughdata units so that the signal will be independent of the stored data.The averaging of the data can be accomplished by window integration orother appropriate low pass techniques. To ensure that in eachintegration window the signal is data-independent, DC free encodingtechniques are used.

In another embodiment of the tracking mechanism the error signalcalculation is accomplished by multiplying (inner product) the timevariation of the data envelope (the read signal) with the reading spotmodulation function. In this scheme the error signal is weighted by thestrength of the modulation, i.e. signal measured when the amplitude ofthe modulation is high contribute more to the error signal. Furtherrefinement of the invention is to include delay compensation before themultiplication between the signal and the modulation.

The tracking errors are used as feedback signals for the servo machinecontrolling the nominal spot position.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, specific embodiments will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 is a graph showing CARS spectra of solid solutions having twodifferent concentration ratios between cis and trans isomers of a givencompound;

FIG. 2 is a schematic illustration of an apparatus according to thepresent invention;

FIG. 3 is a block diagram showing functionally a read/write system foruse with the invention;

FIGS. 4 a and 4 b are pictorial representations showing the effect ofsinusoidally modulating the position of the reading head in the systemof FIG. 2; and

FIGS. 5 and 6 are block diagrams showing details of a tracking systemfor use with the system shown in FIG. 2.

DETAILED DESCRIPTION OF INVENTION

A solid solution of 10% cis-4,4′-dimethoxy-α,α-diciano stillbene(hereinafter compound A) in PMMA was irradiated with collinear laserbeams of 844 and 1037 nm that were focused through a lens to a spotsmaller than 10 μm. CARS signal at a wavelength of 711 mn was detected.The spectra of the signal detected from a solid solution of the cisisomer and of a cis-trans mixture is given in FIG. 1.

FIG. 2 is an illustration of an apparatus 100 according to the presentinvention for retrieving information from a three dimensional storagemedium (hereinafter referred to as “disk”) 102 having an informationcarrying volume (not shown). The apparatus 100 includes two lasers 104and 106, each being a source for a beam of coherent light (110 and 112respectively), and a detector 120 for detecting a beam of coherent light116 which is of different wavelength to the light provided by the lasers104 and 106. The detector 120 transfers an electric signal, createdtherein due to the detection of an optical signal produced by the beamof coherent light 116, to a low noise amplifier 123, which can be a lockin amplifier to a tracking unit 125, so that the data sequence may befaithfully followed, and to an A/D converter 126, whose output is fed toa decoder and error detection and correction (ECC) unit 128(constituting an algorithmic error detector), so that informationencoded in the data sequence may be retrieved.

In the embodiment shown in FIG. 2 there is also a disk mount 202 formounting thereon the disk 102, so that the disk, when mounted, may berotated around its center by a motor 205. The two light sources 104 and106 are connected to two optical fibers 104′ and 106′ arranged to directthe light signals 110 and 112 to a dichroic mirror, 220. The signal 112is transferred through the mirror and the signal 110 is reflectedthereby. Thus, the light may irradiate the disk 102, in such a mannerthat the beams common focus 222 is located within the informationcarrying volume of the disk 102.

The optical unit 101 is mounted on an arm 210, which may rotate aroundan arm axis 230. A lens 240, of the kind used in CD players, ispositioned between the dichroic mirror 220 and the disk 102. Itsposition in the direction parallel to the disk's surface 102′ iscontrolled by the combination of the rotations of the arm 210 around thearm axis 230 and the disk 102 around the disk mount 202. Its position inthe direction perpendicular to the disk's surface 102′ is controlled bya magnetic coil 242, which is also used to control small radial motionsof the lens and thus the position of the common focus 222 within thedisk 102 may be fully controlled. To achieve tracking, the location ofthe common focus is modulated by moving the lens 240 by applying aperiodical electric signal to the magnetic coil 242. The coherent lightsources 110 and 112 in combination with the dichroic mirror 220, thelens 240 and the magnetic coil 242 constitute an optical system 245. Acollecting mirror 250 is positioned to collect the non-linearlygenerated signal 116 and directs it to the detector 120, positioned nearthe arm axis 230, through a filter 152. The laser drivers are not shownin FIG. 2.

The large ratios between the radius of the motion of the optical unit101 around its axis 230 and track radius on the one hand and the size ofthe spot and the distance between adjacent tracks and layers on theother hand allow the approximation that the motion controlled by therotation of the optical unit 101 around its axis is essentiallyorthogonal to the track of the data sequence.

To track, the system is provided with a tracking servo system showngenerally as 125, which feeds a correction signal to the magnetic coil242 for moving the lens 240 under control of the tracking error signalto nominally position the beam spots at the center of the track so thatthe tracking error signal is zero. Coarse motion of spot is achieved bymotion of the optical unit as a whole. Fine motion is achieved by themotion of the lens using the magnetic coil 242.

Although the tracking system 125 shown in FIG. 2 serves to track thedata sequence recorded on the specific disk 102 as described above, itis to be noted that the invention encompasses a novel tracking system,which is well-suited for use in the apparatus 100 described above withreference to FIG. 2 although it is also suitable for use in otheroptical data retrieval systems. Likewise, it is to be noted that othertracking systems may be employed in the apparatus 100.

The improved tracking system according to the invention is describedbelow with particular reference to FIGS. 4 a, 4 b and 5 of the drawings.However, by way of general introduction there will first be describedfunctionally with reference to FIG. 3 a read/write system 300 for a 3-Doptical storage medium 102 having a tracking system. To the extent thatthe read/write system 300 includes components that are common to theapparatus 100 shown in FIG. 2, identical reference numerals will beemployed.

The read/write system 300 comprises a rotary shaft 302 driven by anappropriate driving motor 205 for rotating the optical storage medium102 set thereon, and an optical unit 101 for reading information fromone of the tracks in the optical storage medium 102.

The optical unit 101 comprises semiconductor lasers 104 and 106 forradiating a pair of intersecting light beams having a volume ofintersection that forms a “spot”. Also included within the optical unit101 is an optical system 245 for creating a focused spot whose locationis controlled by an actuator 306, which in the particular embodimentshown in FIG. 2 is constituted by the magnetic coil 242. The opticalunit 101 is driven by a motor 307 so as to produce the required coarseand fine motion of spot described above.

The system further comprises a laser driving circuit 308 for energizingthe semiconductor lasers 104, and 106 to emit the respective laserbeams.

In order to retrieve information from a desired track on the opticalstorage medium 102, the optical focus 222 must be kept on the desiredtrack. To this end, the system is provided with a tracking servo systemshown generally as 125, which feeds a correction signal to the lensactuator 306 for moving the optical system 245 under control of thetracking error signal to nominally position the beam spots at the centerof the track so that the tracking error signal is zero.

FIGS. 4 a and 4 b shows pictorially the effect of sinusoidallymodulating the position of the optical focus 222 in the system of FIGS.2 and 3. So far as the reading spot is concerned it trades data writteninto a continuous linear data track 321 while being subjected to spatialmodulation that shifts its position continually from one side of thetrack to the other. Although the data is stored in tracks, the inventionrelies on the principle that even if the optical focus 222 is slightlyoff-center, data signal will still be read, albeit at reduced intensity.Thus, the further off-center the optical focus 222 is moved, the lowerwill be the magnitude of the data signal.

Thus, with reference to FIG. 4 a, consider the case where the trackingis perfect and the reading spot is symmetrical with respect to the datatrack 321, its position being shown by the sinusoidal curve 322. In thiscase, the average signal will be equal on both sides of the data track321. However, in the case of imperfect tracking as shown in FIG. 4 b,the reading spot is asymmetrical with respect to the data track 321, itsactual line of symmetry being depicted by a dotted center-line 323,shown to right of the data track 321. The signal is inverselyproportional, in perhaps a non-linear fashion, to the distance of thesinusoidal curve 322 from the data track 321. Thus, in FIG. 4 b wherethe sinusoidal curve 322 is offset to the right of the data track, thisresults in a lower signal from samples made when the sinusoidal curve322 is to the right of the center line 323, thus indicating the spot isoffset to the right of the data track 321 and must therefore be shiftedto the left in order to correct the offset.

The tracking operates on the principle that by continually reading thedata and, at the same, continually modulating the position of thereading head, the resulting moving average signal intensity that is readmay be used to indicate to which side, both axially and radially, thereading head is located. This having been determined, the reading headmay then be moved in an opposite direction until it is found to bedisposed symmetrically relative to the track in both axial and radialdirections.

FIGS. 5 and 6 are block diagrams showing functionally details of atracking system 125 that is described in polar coordinates (r, θ, z)defining a position of the beams' intersection in the optical recordingmedium. The tracking system behaves substantially identically for bothradial and axial tracking. A modulator 332 spatially modulates thelocation of the optical focus 222 by a (r, z) modulation signal andfeeds the resulting modulated data signal to the optical storage medium102. The modulation signal itself is fed together with the measured datasignal to an error determination unit 333, whose output is an errorsignal that is fed back to the optical unit 101 to correct the axial andradial offsets thereof. The modulator 332 in conjunction with the errordetermination unit 333 constitutes a tracking error correction unit. Arotation unit 334 provides a continuous change of θ.

FIG. 6 shows in simplified form the principal functionality of the errordetermination unit 333 comprising a first 2-input multiplier 340 towhose first input the (r, z) modulation signal is fed and to whosesecond input is fed the data signal read by optical unit 101 at theposition (r, θ, z) in the optical storage medium 102. An output of themultiplier 340 is fed to a window integrator 341 which integrates theproduct of the data signal with the modulation signal so as to generateat its output a composite (r, z) error signal in the radial and axialdirections.

As described above by way of example with reference to FIGS. 4 a and 4 bof the drawings, the modulation signal can be a sinusoidal function ofthe form m=(sin(ωt), cos(ωt))^(t). The output of the window integratormay then be represented by: err(t) = ∫_(t − T)^(t)m  I(t)𝕕t

The intensity I(t) is inversely proportional in a non linear fashion tothe distance from the center of the track. Thus, when the head is abovethe center of the track the modulation intensity is strongest, and itdecays to zero when the head moves far from it. “T” represents thelength of the time window of the integrator during which the modulatedintensity is averaged. “T” should not be so large that it impactsnegatively on the reaction time and creates distortions; but neithershould it be too low since it is very difficult to construct amechanical scanning system.

However, the modulation signal can be any suitable cyclic function whichserves to move the optical focus 222 on either side, in both axial andradial directions, of the reading spot. Thus, it can be a square wavefunction or any other suitable cycle function. It is assumed that thefrequency of the modulation signal is much lower than the frequency atwhich data is read. The window integrator 341 thus operates as a lowpass filter.

The tracking operates on the principle that by continually reading thedata and, at the same, continually modulating the position of thereading head, the resulting moving average signal intensity that is readmay be used to indicate to which side, both axially and radially, thereading head is located. This having been determined, the optical focusmay then be moved in an opposite direction until it is found to bedisposed symmetrically relative to the track in both axial and radialdirections.

Whilst the tracking method has been described with particular regard toa tracking system for use with a 3-D optical storage retrieval systemwherein data is stored at voxels written in the bulk of the material, itwill be understood that the principles of the invention are equallyapplicable to other kinds of optical storage media where data is storedas a quasi-linear data sequence.

The embodiment illustrated in FIG. 2 is only one of many embodimentsavailable to the artisan when designing an apparatus according to thepresent invention. Some non-limiting examples for variations from thisembodiment include: the lens and the mirror may be replaced by any otheroptical means which is known to bring to the same result, the opticalfibers may be omitted or replaced by any other wave-guide, thecombination of a disk mount and an arm axis may be replaced by any othermeans for controlling the location of the common focus in the plainsparallel to the disk's surface 102′, etc.

It should also be noted that although the preferred embodiment isdirected to retrieval of data, the tracking system according to theinvention is equally suitable for use when writing data to the opticalmedium.

1. A method for retrieving information from a three dimensional storagemedium, the method comprising: using a three dimensional storage mediumcomprising an active medium capable of being in two states, wherein adata unit is represented by the ratio between the concentration of thefirst and second of said two states in a given volume portion of saidmedium and a data sequence is represented by a sequence of such dataunits; irradiating said active medium with light as to concentrate lightflux through a volume portion of said storage medium so as to generatein said volume portion a detectable non-linear optical responsecharacteristic of said concentration ratio, the non-linear opticalresponse being related to a X^((n)) process, where n is greater than 2,allowing for spatially separating the non-linear optical response fromother light signals due to a propagation direction characteristic of thenon-linear optical response satisfying phase matching conditions;detecting said non-linear optical response to retrieve informationstored in said volume portion; and tracking a data sequence forretrieving said data sequence in a reproducible manner.
 2. The methodaccording to claim 1, wherein the active medium includes stillbenederivatives, azobenzene derivatives, or mixtures thereof.
 3. The methodaccording to claim 2, wherein the active medium is embedded in asupporting matrix.
 4. The method according to claim 3, wherein theactive medium is doped into the supporting matrix.
 5. The methodaccording to claim 3, wherein the supporting matrix is a polymer.
 6. Themethod according to claim 5, wherein the active medium is a monomerco-polymerized with the supporting matrix.
 7. The method according toclaim 3, wherein the supportive matrix is transparent to the lightirradiated on it and to the light generated by the non-linear opticalprocess.
 8. The method according to claim 3, wherein the supportivematrix comprises polyethylene, polypropylene, polycarbonate, and/orpolymethylmetacrilate (PMMThe), and/or other transparent polymericmaterial.
 9. The method according to claim 1, wherein the irradiatedlight is focused to a spot having a radius of the order of 30 μm of saidirradiated light or less.
 10. The method according to claim 1, whereinthe intensity of the irradiated light is high enough for the generatedsignal to be independent thereon.
 11. The method according to claim 1,wherein the non-linearly generated light is separated from other lightsignals that may exist in the environment by a filter, prism,monochromator or any other optical element known in the art.
 12. Themethod according to claim 1, wherein the non-linearly generated light isseparated other light signals that may exist in the environment bysatisfying phase matching conditions.
 13. The method according to claim1, wherein the non-linearly generated light is separated from otherlight signals that may exist in the environment by phase sensitivedetection, a low-noise amplifier, a lock-in amplifier, a box-cars, gatedaveraging methods or any electronic method known in the art.
 14. Themethod according to claim 1, wherein the large flux in the volumeportion from which information is retrieved is achieved by focusing twoor more collinear light beams at said volume portion.
 15. The methodaccording to claim 1, wherein the large flux in the volume portion fromwhich information is retrieved is achieved by intersecting two or morefocused light beams, each of which is monochromatic.
 16. The methodaccording to claim 1, wherein the non-linear optical process is a multiphoton fluorescence process.
 17. The method according to claim 16,wherein the non-linear optical process is a two-photon fluorescenceprocess.
 18. The method according to claim 1, wherein the non-linearprocess is selected from Coherent Anti-Stokes Raman Scattering (CARS),Degenerate Four-Wave Mixing (DFWM), Raman Induced Kerr EffectSpectroscopy (RIKES), and/or other four-wave mixing processes.
 19. Themethod according to claim 1, wherein the data sequence is tracked via atracking feedback signal for directing the light spot to a predeterminedvolume portion of the storage medium.
 20. The method according to claim19, further including correcting tracking errors in the optical storagemedium by: (a) directing a reading spot that is nominally focused on toa track in the optical storage medium, (b) continually moving thereading spot in axial and radial directions, (c) receiving a signalhaving an amplitude which varies according to respective offsets fromthe track in radial and axial directions, (d) using the received signalto determine a direction of a respective offset from the track in radialand axial directions, and (e) adjusting a location of the reading spotaccordingly.
 21. The method according to claim 20, wherein directing thereading spot includes directing at least two light sources whose volumeof intersection constitutes the reading spot.
 22. The method accordingto claim 20, wherein moving the reading spot includes modulating aposition of the reading spot with a cyclic function.
 23. The methodaccording to claim 22, wherein the cyclic function is substantiallysinusoidal.
 24. The method according to claim 20, wherein receiving asignal includes: i) reading a data signal with the reading spot, ii)multiplying the data signal by a cyclic modulation signal to form amodulated data signal, and iii) low pass filtering the modulated datasignal.
 25. The method according to claim 24, wherein low pass filteringincludes window integrating the modulated data signal.
 26. The methodaccording to claim 1, further including analyzing and processingdetected signals and retrieving information therefrom.
 27. An apparatus(100) for retrieving information from a three dimensional storagemedium, the apparatus comprising: a mount (202) for mounting thereon athree dimensional storage medium (102) comprising an active mediumcapable of being in two states, wherein a data unit is represented bythe ratio between the concentration of the first and second of said twostates in a given volume portion of said medium and a data sequence isrepresented by a sequence of such data units; at least one source ofcoherent light (104, 106) for irradiating said active medium with lightas to concentrate light flux through a volume portion of said storagemedium so as to generate in said volume portion a detectable non-linearoptical response characteristic of said concentration ratio, thenon-linear optical response being related to a □^((n)) process, where nis greater than 2, allowing for spatially separating the non-linearoptical response from other light signals due to a propagation directioncharacteristic of the non-linear optical response satisfying phasematching conditions; a filter (152) accommodated in an optical path oflight coming from the medium to separate the non-linear optical responsefrom other light signals a detector (120) for detecting said non-linearoptical response to retrieve information stored in said volume portion;and a tracking unit (125) for tracking a data sequence for retrievingsaid data sequence in a reproducible manner.
 28. The apparatus accordingto claim 27, wherein said non-linear optical response is characterizedby predetermined wavelength, polarization, or both of thesecharacteristics.
 29. The apparatus according to claim 27, wherein the atleast one source of coherent light includes an active light source. 30.The apparatus according to claim 29, wherein the active light source isa laser.
 31. The apparatus according to claim 27, wherein the at leastone source for coherent light includes a passive light source.
 32. Theapparatus according to claim 27, further including an algorithmic errordetector (128) for analyzing and processing detected signals andretrieving information therefrom.
 33. The apparatus according to claim27, wherein the tracking unit (125) is adapted for tracking the datasequence via a tracking feedback signal for directing the light spot toa predetermined volume portion of the storage medium.
 34. The apparatusaccording to claim 33, wherein the tracking unit (125) includes atracking error correction unit for correcting tracking errors, the errorcorrection unit comprising: a position modulator (332) for modulating aposition of the reading spot, an error determination unit (333) forreceiving a data signal having an amplitude which varies according torespective offsets from the track in radial and axial directions, and isresponsive to the data signal to determine a direction of a respectiveoffset from the track in radial and axial directions, which offsets maybe fed to the optical unit to correct radial and axial position errorsof the reading spot.
 35. The device according to claim 34, wherein thereading spot is a volume of intersection of at least two light sourcesfocused on the track.
 36. The device according to claim 34, wherein theposition modulator is adapted to modulate a position of the reading spotwith a cyclic function.
 37. The device according to claim 36, whereinthe cyclic function is substantially sinusoidal.
 38. The deviceaccording to claim 34, wherein the error determination unit includes: amultiplier (340) for multiplying the data signal by a cyclic modulationsignal to form a modulated data signal, and a low pass filter (341) forlow pass filtering the modulated data signal.
 39. The device accordingto claim 38, wherein the low pass filter is a window integrator (341).40. A method for correcting tracking errors in an optical storage mediumhaving multiple tracks arranged in different layers of the opticalstorage medium, the method comprising: (a) directing a reading spot thatis nominally focused on to a track in the optical storage medium, (b)continually moving the reading spot in axial and radial directions, (c)receiving a signal having an amplitude which varies according torespective offsets from the track in radial and axial directions, (d)using the received signal to determine a direction of a respectiveoffset from the track in radial and axial directions, and (e) adjustinga location of the reading spot accordingly.
 41. The method according toclaim 40, wherein step (a) includes directing at least two light sourceswhose volume of intersection constitutes the reading spot.
 42. Themethod according to claim 40, wherein step (b) includes modulating aposition of the reading spot with a cyclic function.
 43. The methodaccording to claim 42, wherein the cyclic function is substantiallysinusoidal.
 44. The method according to claim 40, wherein step (c)includes: i) reading a data signal with the reading spot, ii)multiplying the data signal by a cyclic modulation signal to form amodulated data signal, and iii) low pass filtering the modulated datasignal.
 45. The method according to claim 44, wherein step (iii)includes window integrating the modulated data signal.
 46. An errorcorrection device for correcting tracking errors in an optical storagemedium having multiple tracks arranged in different layers of theoptical storage medium that are read by a focused reading spot directedby an optical head to a track in the optical storage medium, the errorcorrection device comprising: a position modulator for modulating aposition of the reading spot, an error unit for receiving a data signalhaving an amplitude which varies according to respective offsets fromthe track in radial and axial directions, and is responsive to the datasignal to determine a direction of a respective offset from the track inradial and axial directions, which offsets may be fed to the opticalhead to correct radial and axial position errors of the reading spot.47. The device according to claim 46, wherein the reading spot is avolume of intersection of at least two light sources focused on thetrack.
 48. The device according to claim 46, wherein the positionmodulator modulates a position of the reading spot with a cyclicfunction.
 49. The device according to claim 48, wherein the cyclicfunction is substantially sinusoidal.
 50. The device according to claim46, wherein the error unit includes: a multiplier for multiplying thedata signal by a cyclic modulation signal to form a modulated datasignal, and a low pass filter for low pass filtering the modulated datasignal.
 51. The device according to claim 50, wherein the low passfilter is a window integrator.